The present invention relates generally to high speed gear pumps and more particularly to inlet ports for gear pump housings.
Gear pumps comprise a species of positive displacement pumps in which two generally equally sized intermeshed gears rotate to convey a viscous liquid. The gears are mounted for rotation with their teeth intermeshing in a housing having an inlet port at one side of the intermeshed teeth and a discharge port on an opposite side of the intermeshed teeth. Rotation of the intermeshing gears draws in liquid through the inlet port. Inside the housing, the liquid is carried by each gear in gear pockets formed between adjacent gear teeth and the close clearance sealing zone within the housing. The liquid from each gear pocket is joined together at the discharge port and pushed from the housing. Rotation of the gear teeth away from each other at the inlet produces an increase of volume as the fluid is drawn into the gear pockets resulting in a pressure drop that draws liquid into the inlet port. Conversely, rotation of the gear teeth toward each other at the discharge port produces a decrease of volume in the pump housing that results in a pressure increase that pushes the liquid out the discharge port. The inlet port and discharge port are substantially isolated from each other by the intermeshing of the gear teeth between the inlet port and discharge port and engagement of the gears with the surfaces of the housing. Gear pumps are commonly used in aerospace applications for fuel and lubricating systems.
Operation of the gear pump at elevated speeds for aerospace applications increases the inlet dynamic pressure, which can cause cavitation erosion. In order to facilitate rotation of the gears within the housing, side bearings comprising flat plates are mounted adjacent the flat faces of the gears. Cavitation erosion frequently occurs on the side bearing faces adjacent to the intermeshed gear teeth, at the center of the gearteeth, and on the pump housing at the inlet port where the gear tooth tips enter the close clearance sealing zone with the housing. Cavitation erosion affects sealing of the gears with the side bearings and the pump housing. Cavitation erosion is caused by air trapped in the liquid being pumped by the gear teeth. Specifically, air and fluid vapor bubbles are introduced into the liquid as the gear teeth come out of mesh at the inlet port. As air and vapor within the liquid comes out of solution due to the vacuum created in the expanding gear mesh, the bubbles are driven to the center of the gear mesh by flow entering through passages in the bearing faces at the gear side faces. The fluid experiences a limiting drop in pressure as the velocity increases to fill the vacuum in the gear mesh. As the gear teeth continue to rotate out of mesh, the liquid pressure instantaneously increases at the inlet port due to a “hydraulic front” that causes the air to collapse back into solution. The implosion of the air produces a pressure shock that causes cavitation and damage to the pump components, which can be costly to repair or replace.
Cavitation damage is currently a limiting design factor in gear pumps used as fuel pumps in aircraft. Specifically, it is always generally desirable to reduce the size and weight of components used in aerospace applications. Smaller gear pumps can be used to achieve the desired output if operated at higher speeds. However, high speed operation of a pump decreases the inlet static pressure for a given fixed inlet total pressure with the aforementioned high inlet dynamic pressure. Reduced inlet static pressure in the expanding mesh introduces additional air bubbles into the liquid. Low pressure air travelling at high velocities can cause cavitation damage of the pump housing near the inlet. It is, therefore, desirable to eliminate cavitation damage produced during operation of high speed gear pumps.